Fiber gratings are under intense investigation as reflecting or dissipating elements in photonic components. It is desirable to have the grating not respond to changes in ambient temperature. A simply supported fiber with a grating written in it will shift in center wavelength as it is heated, as thermal expansion changes the period of the grating. Moreover, changes in temperature also cause shifts in the refractive index of the grating material.
In the prior art, short period gratings have been athermalized using beta-eucryptite, which has a negative coefficient of thermal expansion. In other words, beta-eucryptite shrinks as the temperature increases and expands as the temperature decreases. A fiber with a grating written on it is placed in tension and attached to the beta-eucryptite substrate, with the fiber being attached to the substrate at two points along its length. The fiber has a positive coefficient of thermal expansion. As the ambient temperature increases or decreases, the beta-eucryptite substrate shrinks or expands tending to compensate for the fiber's tendency to physically expand or shrink and its change in refractive index with change in temperature. The substrate is preferably chosen and designed such that the shrinking as temperature increases or the expansion as temperature decreases of the substrate exactly cancels the intrinsic response of the fiber to temperature changes.
The beta-eucryptite substrates of the present state of the art, however, presently require a hermetic package to survive the damp heating which may occur in a variety of typical real world environments. The beta-eucryptite material is microcracked, and the presence of moisture causes a hysteresis in the expansion and contraction response of the substrate. The necessity of enclosure in a hermetic package increases expense. The hermetic package also requires additional space and decreases the suitability of beta-eucryptite substrates in space critical applications.
There exists, therefore, a need in the art for a technique of fiber grating athermalization using robust materials with positive coefficients of temperature expansion. A similar need exists for other temperature sensitive components, such as an external cavity mode-locked laser which requires an athermalization package.